Cosmic collision appears to upend dark-matter theory

Telescopes atop Mauna Kea have captured a massive collision of groups of galaxies -- described by scientists as a cosmic "train wreck" -- that deepened the mysteries surrounding the phenomenon known as dark matter.

The observations were made by the Subaru and Canada-France-Hawaii telescopes on the Big Island peak, as well as the Chandra X-ray Observatory, according to a news release last week from the participating universities.

The collision, involving giant galactic clusters, has produced a chaotic scene never before witnessed, scientists said.

Typically, galactic clusters contain not only galaxies, but dark matter and clouds of gas. Current theory holds that the three elements should stick together even during violent collisions.

But the images from Mauna Kea and Chandra show a dark-matter core that contains hot gas but no bright galaxies and also a group of galaxies that has no dark-matter core.

"It blew us away that it looks like the galaxies are removed from the densest core of dark matter," remarked Hendrik Hoekstra of the University of Victoria in British Columbia.

His colleague Arif Babul added, "The observation of this group of galaxies that is almost devoid of dark matter flies in the face of our current understanding of the cosmos. Our standard model is that a bound group of galaxies like this one should have a lot of dark matter."

Possible explanations include some kind of gravitational slingshot effect or an as-yet-unknown interaction between dark-matter particles.

Only about 4 percent of the universe is visible in the form of stars, galaxies and hot gas, scientists estimate. About 23 percent is believed to be dark matter, the composition of which remains unknown. The rest is believed to be an equally mysterious entity known as dark energy.

Optical telescopes can infer the presence of dark matter by its subtle light-bending effects on distant galaxies.